1. Field of the Invention
The present invention relates to a spark plug for an apparatus for detecting an ion current without generating spike-like noise on the ion current.
2. Related Arts
A conventional spark plug 3 for an ion current detecting apparatus for detecting an ion current as shown in FIG. 1, has a cylindrically shaped insulator 32, a cylindrically shaped metallic body 31 retaining the insulator 32 therein, and a center electrode 33 and a stem portion 34 retained in the insulator 32. Further, a ground electrode 35 is fixed to an end portion 311 of the metallic body 31 to face the end portion 331 of the center electrode 33 through a discharge gap 38. The insulator 32 has a ramp portion 32a at a portion corresponding to the other end portion 312 of the metallic body 31 and a small diameter portion 323 on the side of the end portion 322 thereof (on the upper side in FIG. 1) with respect to the ramp portion 32a. The metallic body 31 is fixed to the insulator 32 by caulking the end portion 312 thereof along the ramp portion 32a of the insulator 32.
To operate the spark plug 3, the end portion 3b of the spark plug 3 having the ground electrode 35 and the center electrode 33 is inserted into a combustion chamber of an internal combustion engine and a high voltage of approximately 10 kV to 35 kV is delivered to the spark plug 3. Accordingly, a spark discharge occurs between the ground electrode 35 and the center electrode 33 in the discharge gap 38 so that an air-fuel mixture in the combustion chamber is ignited. The burning of the air-fuel mixture is accompanied by electrolytic dissociation to generate ions, so that ion current flows between the center electrode 33 and the ground electrode 35 (that is, the metallic body 31). Recently, detecting the burning state of the air-fuel mixture in the combustion chamber and knocking of the engine by detecting the ion current has been studied. The ion current is usually detected by an ion current detecting apparatus.
The waveform of the ion current detected by the ion current detecting apparatus is shown in FIG. 2. Generally, when the ion current detecting apparatus detects an ion current having an waveform including a build-up portion with a rise height of H and rise duration of more than a specific duration T, it is judged that the air-fuel mixture is burning. When the burning of the air-fuel mixture stops, the ion current is not generated, so that the above-mentioned build-up portion is not detected. Just before the air-fuel mixture is ignited, the ions are generated in the discharge gap 38 so that the build up of the ion current is detected. An oscillating waveform K of the ion current shown in FIG. 2 occurs in response to the knocking of the engine, thereby detecting the knocking of the engine to control the timing of igniting the air-fuel mixture.
However, when spike-like noise N shown in FIG. 2 is generated on the waveform of the ion current, the spike-like noise N is likely to cause a false detection by the ion current detecting apparatus. For example, the ion current detecting apparatus is likely to judge the spike-like noise N as the oscillating waveform K, thereby resulting in misjudgment that the knocking of the engine is generated. In a full-open state of a throttle valve of the engine, the pressure in the combustion chamber is high in comparison with the full-closed state of the throttle valve, so that the required voltage applied to the spark plug 3 becomes high. In this case, the spike-like noise N is frequently generated on the ion current. Thus, the ion current detecting apparatus has a tendency to make the false detection frequently in the full-open state of the throttle valve.